The present invention relates to a four-wheel drive motor vehicle of a type having a center differential between the front and rear wheels, and means for restraining or locking the center differential. More specifically, the present invention relates to a control system for unlocking a center differential of a four-wheel drive system in accordance with a steering angle of the steerable wheels.
A four-wheel drive vehicle is superior in ability of hill climbing and ability of rough road driving. If, however, all four wheels are always driven at the same speed, a four-wheel drive vehicle cannot be turned smoothly. During a turn with a large steering angle of steerable wheels, the front wheel of an inner or outer side must travel along a circle having a large radius and the rear wheel of the same side must travel along a small circle. Because of this difference in turning radius, there arises a large difference between a rotation speed (an average rotation speed, to be exact) of the front wheels and a rotation speed (an average rotation speed) of the rear wheels. As a result, the steering becomes heavy, the tendency to understeer is increased, and the vehicle cannot be turned without abnormal tire friction (called tight corner brake) which tends to brake the vehicle and cause an engine stall.
To overcome these handling and tire friction problems, some four-wheel drive systems use a center differential between front and rear wheels. If, however, one of the four wheels run on a muddy place, the center differential coacts with a rear or front differential between right and left wheels, so that the torque cannot be transmitted to the other three wheels, and the vehicle cannot escape from the muddy place.
In view of this problem, some four-wheel drive systems are further provided with means (a lockup mechanism or a non-slip differential mechanism) for locking the center differential. One example is shown in Japanese patent provisional publication No. 57-114727.
However, a four-wheel drive vehicle of this type is very difficult to control especially for the average driver having no special skill. If a four-wheel drive vehicle of this type is turned at a large steering angle without unlocking the center differential, abnormally great torques are exerted on ends of front and rear propeller shafts and axle shafts which transmit power to the front wheels and the rear wheels. Therefore, the steering of the vehicle becomes heavy, and there arises a danger of damage to the propeller shafts and axle shafts. Furthermore, the front wheels and the rear wheels slip in the opposite directions during a turn, so that the tendency to understeer is increased and the effect of tight corner brake is produced. The average driver cannot understand the reason for such an abnormal behavior of the vehicle, and cannot negotiate a corner smoothly.